Target holders, multiple-incidence angle, and multizone heating for BNNT synthesis

ABSTRACT

In the synthesis of boron nitride nanotubes (BNNTs) via high temperature, high pressure methods, a boron feedstock may be elevated above its melting point in a nitrogen environment at an elevated pressure. Methods and apparatus for supporting the boron feedstock and subsequent boron melt are described that enhance BNNT synthesis. A target holder having a boron nitride interface layer thermally insulates the target holder from the boron melt. Using one or more lasers as a heat source, mirrors may be positioned to reflect and control the distribution of heat in the chamber. The flow of nitrogen gas in the chamber may be heated and controlled through heating elements and flow control baffles to enhance BNNT formation. Cooling systems and baffle elements may provide additional control of the BNNT production process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/659,149, filed Jul. 25, 2017, which is a divisional of U.S.application Ser. No. 15/053,200, filed Feb. 25, 2016, now issued as U.S.Pat. No. 9,745,192, which is a continuation of International ApplicationNo. PCT/US2015/058615, filed Nov. 2, 2015, which claims the benefit ofU.S. Provisional Application No. 62/074,002, filed Nov. 1, 2014, U.S.Provisional Application No. 62/074,004, filed Nov. 1, 2014, and U.S.Provisional Application No. 62/194,972, filed Jul. 21, 2015. Thecontents of each application are expressly incorporated by reference.

STATEMENT REGARDING GOVERNMENT SUPPORT

None.

FIELD

The present disclosure generally relates to generating boron melts andenhancing the synthesis of boron nitride nanotubes.

BACKGROUND

Generally, BNNT structures may be formed by thermally exciting a boronfeedstock in a chamber in the presence of nitrogen gas at an elevatedpressure. Unlike carbon nanotubes (CNTs), U.S. Pat. No. 8,206,674 toSmith et al., incorporated by reference in its entirety, indicates thatBNNTs form without the presence of chemical catalysts, and preferably atelevated pressures of about 2 atm to about 250 atm. CNTs, on the otherhand, typically require the presence of chemical catalysts such as metalcatalysts. It has been shown that BNNTs do not form in the presence ofsuch catalysts, indicating that the formation of BNNTs is fundamentallydifferent than the formation of CNTs.

Most contemporary BNNT synthesis methods have severe shortcomings,including one or more of having low yield, short tubes, discontinuousproduction, poor crystallinity (i.e., many defects in molecularstructure), poor alignment and high levels of boron impurities. Althoughthere is no agreed upon standard in the scientific literature, the term‘high quality’ BNNTs generally refers to long, flexible, molecules withfew defects in the crystalline structure of the molecule.

BRIEF SUMMARY

This disclosure relates to apparatus, systems, and methods for theproduction of BNNTs where the source of boron is a ball of melted boronin a nitrogen environment and the boron ball is supported on a thin aninterface layer of material that provides the required thermalinsulation from the surrounding support structures. In addition, theheat being supplied to the process can come from multiple zones in theprocess including multiple directions in the case where the heat sourceis directional.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates heating zones in an embodiment of an apparatus forheating a boron feedstock to a boron melt.

FIGS. 2A, 2B, 2C and 2D show an embodiment of an apparatus for heating aboron feedstock to a boron melt with several techniques for managing thenitrogen gas flow.

FIG. 3 illustrates another embodiment of an apparatus for heating aboron feedstock to a boron melt with supplemental heat provided to thenitrogen gas.

FIGS. 4A, 4B and 4C illustrate another embodiment of an apparatus forheating a boron feedstock to a boron melt with another embodiment ofsupplemental heat to the nitrogen gas.

FIG. 5 illustrates the use of mirrors in an embodiment.

FIG. 6 shows an embodiment of an apparatus splitting a single laser beaminto several beams for generating a boron melt.

DESCRIPTION

Described herein are methods and apparatus for synthesizing boronnitride nanotubes, such as through a high temperature, high pressureprocess. The following description should not be taken in a limitingsense, and is made for the purpose of illustrating embodiments of thepresent approach.

The synthesis of boron nitride nanotubes (BNNTs) by a high temperature,high pressure (HTP) process, also known as the pressurizedvapor/condenser method (PVC), may involve heating boron to a liquidboron melt. A boron feedstock may be heated to form a liquid boron meltby heating all or a portion of the boron feedstock to a temperature nearboron's smoking point. In most HTP processes, the boron feedstock isheated to a boron melt in a nitrogen atmosphere under elevated pressure,typically greater than 2 atmospheres, such as about 2 atm to about 250atm, and in some embodiments about 2 atmospheres to about 12atmospheres. This disclosure describes heating the boron feedstock(sometimes referred to as the boron ball, even though the feedstock maynot be spherical in some instances) to generate a boron melt in an HTPprocess, terms of three heating zones, as shown in FIG. 1. Thisapplication refers to these heat zones as preheat-support zone 13, boronmelt zone 12, and BNNT growth zone 11. It should be noted that in someembodiments, there may be a gradual transition and/or overlap betweenzones. Also, any transition and/or overlap between zones may changeduring operation at time continues, such as, for example, from start-upto continuous production as near steady-state conditions.

It should be noted that the processes and systems described herein donot apply to the formation of carbon nanotubes (CNTs). HTP BNNTsynthesis processes and systems involve forming a liquid material from aboron feedstock in more or less steady state and at very hightemperature, the boron in a high pressure gaseous environment, andnitrogen, such that the process produces combination of the liquidmaterial and the gas, without involving catalysts or other elementalspecies. On the other hand, CNTs synthesis usually requires metalcatalyst or other elements such as hydrogen that do not end up in theCNTs except as impurities. Certain arc discharge and laser process willmake limited quantities of CNTs, usually in vacuum, low pressureenvironments of hydrocarbon gases or inert gases. As a final example ofthe differences between the synthesis of BNNT and CNT, one of these CNTsynthesis processes involve having a steady state ball of liquid carbon;note this would minimally require a temperature of 4,300 C.

In embodiments of the present approach, the boron target for the PVCmethod is a molten droplet or ball of boron, also referred to as theboron melt, formed from an initial boron feedstock. After formation, theboron melt may be heated by a heat source, such as one or more laserbeams, to generate boron vapor. Boron vapor in the presence of nitrogengas at an elevated pressure drives the BNNT synthesis process. In earlyexperiments, the boron melt was held in place at the tip of a solidboron or boron nitride rod using surface tension/adhesion. The inventorsdetermined that such an approach limits the amount of laser power orheat that can be applied to the boron target, because the boron melttarget is prone to detach from the rod. As a consequence, supporting theboron melt limited the BNNT synthesis and production of high qualityBNNTs at high yields. Described below are various embodiments of boronmelt target holders that overcome this limitation, and significantlyincrease the BNNT synthesis. For example, prototype apparatus employingthe present approaches have shown BNNT yield increases of over 600%, ascompared to prior apparatus.

FIG. 1 illustrates the heating zones in embodiments of an apparatus forheating a boron feedstock to a boron melt. As shown in this figure,nitrogen gas may be flowing from bottom to top, and heat may be suppliedfrom one or more sides (e.g., left, right, into the drawing, and out ofthe drawing). The preheat-support zone 13 includes physical support forthe initial boron feedstock (e.g., at start-up), and also for the boronmelt 16 generated after and during heating. It should be appreciatedthat the target holder 17 may be sized to support the particular initialboron feedstock selected, as well as the boron melt 16 formed from theinitial boron feedstock, and also should include one or more supportstructures (not shown) to maintain the location of the target holder 17within the chamber 15 of the apparatus. The boron melt zone 12 refers tothe zone where heat may be introduced to the apparatus chamber 15 andheat the initial boron feedstock to raise the boron feedstocktemperature sufficiently high, e.g., to raise the temperature of theboron melt 16 to its smoking point. When the temperature of the boronmelt 16 has reached its smoking point, the boron feedstock releasesboron and boron-nitrogen molecules into the BNNT growth zone 11. In BNNTgrowth zone 11, the self-assembly process forms BNNTs.

FIG. 1 also shows interface zone 14 between the boron melt 16 and thetarget holder 17. The interface zone 14 provides the thermal insulationof the physical support from the boron melt 16. Thermal insulation isuseful because the boron melt 16 must reach and may exceed temperaturesof 2076 C. In embodiments of the present approach, there may be atemperature gradient across the interface zone 14, such that the targetholder 17 remains below its melting point, or a temperature at which itmight chemically react with the molten boron melt 16, and yet the boronmelt 16 can remain molten. Portions of the boron melt 16 are higher intemperature to reach the smoking point. Preferably, the target holder 17is cooled sufficiently to prevent its melting. In addition to thecooling methods described below, those of skill in the art may befamiliar with known methods for cooling support structures in heatedenvironments. The size of the target holder 17, including its length andcross sections, should be matched to the amount or rate of heat flowingto the target holder 17 from the interface zone 14, and the amount orrate of cooling provided. The quantities of heat flowing into the targetholder may also depend on the size of the boron melt 16, the amount ofheat being supplied to the boron melt 16, and the amount of cooling fromthe nitrogen gas within the overall containment volume of the chamber15.

The interface zone 14 material may be boron nitride and/or a thin layerof nitrogen gas. Boron nitride melts at 2973 C, and can support theboron melt 16 if the target holder 17 is of appropriate size, material,and cooled as discussed above. A boron nitride interface zone layer 14may be formed by maintaining a temperature gradient between the initialboron feedstock and the target holder 17, and heating the initial boronfeedstock. The boron nitride interface zone 17 forms as the boronfeedstock is heated to near its melting point. However, in someembodiments the boron nitride interface zone 14 layer may be inserted atthe start of the process. For example, a layer of boron nitride may bedeposited on the target holder 17, such as prior to placing the initialboron feedstock on the target holder 17, or before heating begins.

The nitrogen gas contained in the chamber 15 can also be utilized toprovide both heat and cooling in the process. The nitrogen gas, or aportion thereof, can be heated by supplemental heating elements in allthree zones, 11, 12 and 13. The target holder 17, boron melt 16, and theself-assembly zone 11 can be heated or cooled by controlling the flow ofnitrogen gas in those zones. Heat sources, baffles, and water cooledelements can be utilized separately or in combinations, to achieve thedesired nitrogen gas flow patterns and heat exchanges.

FIGS. 2A-2D illustrate several embodiments of a target holder. FIG. 2Ashows a target holder having a cooled post 21 supporting the boron melt26. Post 21 may include cooling elements, such as water cooling system23. The boron feedstock contact surface 22 a of the target holder 21 inthis embodiment has a concave shape so as to support the boron melt 26.It should be appreciated that other embodiments may use the shape ofcontact surface 22 a to support the boron material as desired. Thematerial for the post 21 may be chosen from a group of materials withhigh thermal conductivity, low reactivity with boron, and a relativelyhigh melt temperature. These materials include, for example, copper,molybdenum, and tungsten, as well as alloys of these materials. Asdiscussed above, a thin layer of boron nitride may be included or formedbetween the boron melt 26 and the post 21, to insulate the boron melt 26from the post 21, and minimize any reaction of the boron material withthe target holder post 21. Target holder 21 may include one or moresupport structures 21 a, 21 b, and 21 c that provide physical supportand stability to e target holder 21, and may also serve as heat transferelements as discussed below. In this embodiment, support structures 21a, 21 b, and 21 c form a pyramid-shaped structure and may have acylindrical hollow middle region for target holder 21. It should beappreciated that other geometries may be used for support structureswithout departing from the present approach. Support structures mayoperate as heat sinks to increase the rate of heat transfer from thetarget holder 21 a, 21 b and 21 c. Additional heat sinks, as are knownin the mechanical arts, may be incorporated to the target holder 26.

FIG. 2A shows an example where the boron feedstock is a ball 26supported by a water-cooled target holder 21. The water-cooling system23 prevents the holder 21 from melting. In this embodiment, coolingchannels 23 a are built into support structures 21 a, 21 b, and 21 c.The cooling system 23 may pump cool water into the channels 23 a, suchthat the contact between target holder 21 and support structures 21 a,21 b, and 21 c, allows heat transfer from target holder 21. It should beappreciated that channels may be built into the target holder 21 andpost, and/or other structures of the apparatus (not shown). As discussedabove, the amount of cooling needed depends on the amount of heat comingin from the boron melt 26 via the interface zone 14. The amount ofcooling is preferably controlled so that the interface zone 14 is notoverly cooled, and at the same time there must be sufficient coolingsuch that the target holder does not melt or chemically react with theboron.

FIG. 2B illustrates a holder which includes a flow baffle 24 which maybe used to control and/or deflect the flow of cold nitrogen gas aroundthe target, reducing quenching of the nanotube formation process. Inthis embodiment, baffle 24 resembles a disc-like ledge extending beyondthe outer edge of the target holder 21. It should be appreciated thatother geometries may be used for baffling on and/or around the targetholder 21. In some embodiments, it is beneficial to keep the nitrogengas flow 22 near the boron ball 26 as steady, i.e. as laminar, aspossible. Thus, some embodiments of a target holder 17 include a flowbaffle 24 and flow tube 27, such as a metal or ceramic cylinder or othershape, that can withstand the temperature of the local environmentinserted around the heating element with adjustable gaps 28 a, 28 b, and28 c, to the surrounding support materials to manage the convective flowof the nitrogen gas and thereby assist in extending the region of thegrowth zone 11. In this embodiment, flow tube 27 resembles a cylindricalstructure around a portion of the target holder extending beyond supportstructures 21 a-21 c. Gap 28 a is present between an upper edge of tube27 and baffle 24. Gap 28 b is present between a lower edge of tube 27and support structures 21 a-21 c. Gap 28 c is present between the outerradial edge of the target holder and the tube 27, and represents an areathrough which a portion of nitrogen gas may flow. The gaps may be sizedto provide a desired nitrogen gas flow and heat transfer profile for aparticular apparatus. Flow control baffle 24 is made from a materialwith a high service temperature (above about 2000 C) such as tungsten ora ceramic such as boron nitride, silicon carbide or zirconia. The flowcontrol baffle 24 can in some implementations be built into the supportpost of target holder 21 and in other implementations can beindependently supported.

FIGS. 2C and 2D illustrate further embodiments of flow control devices.Generally, these configurations use a ledge 25 having back facing step25A with a sharp trailing edge 25B to prevent or limit mixing of coldambient gas 29 with the hot process gas 210, e.g., nitrogen gas that hasbeen heated in the process, and limit subsequent quenching of thenanotube formation process. FIG. 2D shows a close-up view for portion ofan embodiment similar to FIG. 2C, having an extended target holder 21for improved gas pre-heating from the gas having additional surface areato pre-heat. The sharp trailing edge 25B in this embodiment extendsfarther with respect to the direction of nitrogen gas flow. Although thedrawings show flow control devices 25 on opposite sides of the contactsurface for simplicity, it should be understood that flow controldevices may extend around the entire outer edge of the target holder. Asone skilled in the art should appreciate, many target holderconfigurations and flow control devices can be envisioned, such as usingbackward facing steps, splitter plates, and airfoils, for example, toimplement the present approach. Preferably, embodiments will employ oneor more flow control devices that operate at a high temperature andprotect the nanotube formation zone from premature cooling by ambientgases.

Some embodiments of the present approach may include a nitrogen gaspreheating element. Preheating nitrogen gas may extend the BNNT growthzone, as shown in the comparison between original growth zone 310 andextended growth zone 320. Embodiments featuring a nitrogen gas heatingelement may incorporate a target holder 35 extending from a sidewall ofthe chamber 300. An alternative support for the boron melt is shown inFIG. 3 where a water cooled target holder post 31 supports the targetholder 35 and the boron melt 36. This configuration demonstrates anembodiment of a process to preheat the nitrogen gas flowing around theboron melt 36. It should be noted that flow and temperature controlaspects described above may be incorporated into such embodiments. Inthis embodiment, nitrogen gas enters the chamber 30 from the bottom ofthe drawing, and proceeds toward the top of the drawing. One or morenitrogen gas flow channels 300 may be provided for introducing thenitrogen gas flow to the chamber 300. Heating in the preheat-supportzone in this embodiment may be provided by a heating coil 32, althoughother heat sources as are known to those of skill in the art may beused. It is preferable to keep the flow near the boron ball 36 assteady, i.e. as laminar, as possible, so a flow baffle or tube 37 may beinserted around the heating element. Some embodiments of a baffle ortube 37 may include an adjustable gap 38 above a surface of chamber 30,to limit the convective flow of the nitrogen gas and thereby assist inextending the region of the growth zone 39. In this embodiment, the coolnitrogen gas enters the baffle region or tube 37 at the adjustable gap38. Gap 38 may be altered in terms of height and/or width to generatethe desired nitrogen gas flow and heating profiles. The nitrogen gas isthen heated by the coil 32 as the nitrogen gas convectively flows overthe coil 32 and towards the target holder 35 and boron melt 36. Someembodiments may connect heating coil 32 to water-cooled connectors 33and 34 if the currents are sufficiently high to require cooling of theconnectors. The heating coil may be powered by, for example, directcurrent external supply (not shown). The heat could also be supplied byusing AC current using a metal wire, post or other conductor geometryeffective in conveying the heat to the local nitrogen gas. In someembodiments, the heating coil 32 may be formed from or including one ormore refractory materials, such as, for example, molybdenum or tungsten.Generally, it may be desirable to avoid rapid reactions between therefractory material and the nitrogen gas. Otherwise, the process mayquickly reduce the coil's effectiveness.

FIGS. 4A, 4B, and 4C illustrate embodiments of an apparatus forsynthesizing BNNTs, including a target holder 46 and nitrogen gasheating elements 41 and 42. In the embodiment shown in FIG. 4A, theheating elements 41 and 42 include graphite heater elements 41 and 42.Graphite provides certain advantages as it has minimal chemical reactionto the heated nitrogen gas compared to some metals. In this embodiment,graphite heater elements 41 and 42 include a ceramic spacer 43 betweenparallel graphite heater elements 41 and 42. Graphite heater elements 41and 42 may be connected to electrodes as shown in FIG. 3 and the may becooled, such as, for example, through cooling clamps. Spoon 44 may beconfigured to hold the boron melt 46, such that heat from the graphiteheater elements 41 and 42 transfer to the boron melt 46. Spoon 44 may becooled, such as through connection to a cooling strut 45. Flow baffling,not shown, such as the flow baffle 24 as describe for FIG. 2 can beadded. It should be appreciated that variations in these embodiments arepossible without departing from the principles described herein. Thebaffles 24 and 25 shown in FIG. 2 and the tube 37 shown FIG. 3, can alsobe added to the embodiment shown in FIG. 4 to beneficially affect thegrowth zone above the boron melt 46.

For example, FIGS. 4B and 4C show embodiments of an apparatus forheating a boron feedstock to a boron melt in which the heating elementsinclude grafoil heater elements 47 and 48. In the inverted V geometryshown in FIG. 4B, the electric current to heater element 47 is fed fromthe bottom through electrodes 410, which may be, for example, copperelectrodes, to produce hot zone 450. In the embodiment shown in FIG. 4C,the current flows from the support post 460 to the bottom 470, producinghot zone 480, In both geometries of heater elements 47 and 48, the shapeand thickness of the grafoil may be adjusted to maximize the heating ofthe surrounding nitrogen gas near the boron melt 46. The grafoilarrangements 47 and 48 allow for tailoring of the electrical resistanceof the grafoil such that resistance heating creates the highesttemperature of the grafoil nearest the location of the boron melt 46. Asone skilled in the art of working should appreciate, a large variety ofgrafoil geometries can be created such as to optimize the local heatingof the surrounding nitrogen gas as it convectively flows towards theboron melt 46 and on to the BNNT growth zone 11 as seen in FIG. 1 abovethe boron melt 46. Flow baffling, not shown, such as the flow baffles 24and 25 as describe for FIG. 2 can be added.

The embodiments shown in FIGS. 2, 3 and 4 may use supplemental heatingin the preheat-support zone to preheat the nitrogen gas feeding into theboron melt. The goal is to optimize the distribution of heat going intothe BNNT formation process such that the BNNTs have the maximumresidence time in the growth zone 11 as seen in FIG. 1. It should beappreciated that the particular configuration of an apparatus andprocess will depend on the size, geometry, and configuration of theparticular embodiment. Preferably, the preheating of the nitrogen gas inzone 13 does not result in the formation of undesired boron nitridecompounds such as amorphous BN, short BNNTs, i.e. less than a fewhundred nanometers for a given segment, with very large numbers ofwalls, i.e. 10 and greater, and large hexagonal boron nitride, h-BN,particles that do not lead to the formation of high quality HTP BNNTs,These undesirable conditions can occur if the heating is insufficient.By controlling the heat distribution and by the addition of baffling tomanage the nitrogen gas flow so as to keep it in the laminar or steadyregime, adding heat in zone 13 results in additional heat going into theboron melt in zone 12 and in additional residence time for the growth ofHTP BNNTs in zone 11.

As described in related applications International Patent ApplicationNo. PCT/US14/063349, and International Patent Application No.PCT/US15/27570, the contents of which are expressly incorporated byreference, heat may be applied directly to a boron feedstock and, aftera boron melt is generated, to the boron melt. FIG. 5 illustrates a boronmelt 56 formed on a post 57 (which may or may not be independentlyheated, as described above). Preheat-support zone 53, boron melt zone52, and BNNT growth zone 51, are shown in this embodiment as separate,but it should be appreciated that they may overlap; the interface zone58 is between the preheat-support zone 53 and the boron melt zone 52.The heating of the boron melt 56 in the boron melt zone 52, can beachieved by numerous means, including, for example, one or more oflasers, direct arc, and inductively coupled plasma (ICP). The heatingelement(s) may be selected for various reasons, including, for example,heat element cost, heating efficiency, heating requirements, the size ofthe boron feedstock and/or boron melt 56, and the nitrogen flowconditions. For example, if the boron melt 56 is sufficiently large torequire mechanical support as discussed above for FIGS. 2, 3, and 4,then an ICP might not be the most efficient heating element. Additionalheat can be supplied to the boron melt zone 52 through additionalheating elements. In embodiments using laser heating and black bodyreflected heating, additional heat may be supplied to boron melt zone 52through the addition of mirrors 54 a and 54 b. The resultant improvementinvolves heating the target from multiple angles to create multiplepoints from which boron vapor is liberated from the boron melts moltensurface.

Other implementations of this concept can readily be achieved usingcombinations of other beam splitters and/or beam-shaping mirrors, forinstance paraboloids, ellipsoids, conic sections, rings, and multipleflat mirrors, or combinations thereof. Fiber optic coupling could alsobe used to create multi-angle pumping of the target.

In embodiments using laser heating to generate and maintain the bornmelt 56, mirrors 54 a and 54 b may be used to direct portions of a laserbeam that miss or reflect from the boron melt 56, back onto the boronmelt 56 and thereby heating multiple areas on the boron melt 56 via asingle laser. FIG. 6 shows from (A) a side view and (B) from a top view,an example where a single laser beam 61 is focused by a lens 62 or othersuitable optics such that the focused beam 63 or intercepts the boronmelt 66 but a large fraction of the incident beam 61 misses the boronmelt, is then reflected 65 by a mirror 64 back on to the boron melt. Inmore detail, the laser beam travels from left to right through aplano-convex cylindrical lens 62 which converts it to a thick ‘lasersheet.’ The laser sheet partially passes over the boron melt 66 on theway to a plano concave reflector 64. A portion of the laser sheet isabsorbed on the left surface of the target (indicated by ‘A’), and thereminder of the sheet strikes the reflector where it illuminates theright side of the target (indicated by ‘B’ and ‘C’) in the Top View ofFIG. 6. Thus the target is illuminated in 3 separate regions, greatlyincreasing the generation of boron vapor and the production of BNNTs. Atelevated power levels for some size boron melts 66 and laser poser, themirror 64 must be water cooled. The geometry and position of a mirror,as well as the number of mirrors, may depend on the relative size andshape of the apparatus' chamber, as well as the amount of desired heatfrom redirected beams. In addition, a mirror 54 a and 54 b can reflectblack body radiation from the boron melt 56 back on to the boron melt56, increasing the overall heating efficiency. An importantconsideration in the laser beam or laser beams is that for someembodiments the power levels are sufficient such that for some sizeboron melts 66 the momentum transfer of the laser light 63 and 65 on theboron melt will push the boron melt 66 off the support post or 57 orspoon 44 unless the forces from the light are appropriately balancedaround the circumference of the boron melt 66.

As one of ordinary skill in the art should appreciate, the multiplebaffles and flow control elements such as 24, 28 and 38 when combinedwith heat sources such as lasers 61 and ICP can also be used tomanipulate the heat including providing additional heat to the nanotubeself-assembly region 11 and 51.

The mechanical structures, water cooling for the coils and surroundingsurfaces, the nitrogen pressure chamber and the systems to harvest theBNNTs are not shown in the FIGS. 1-6. As one of ordinary skill in theelectromechanical design and construction in the art of working elevatedpressure electromechanical systems should appreciate, a very diversenumber of arrangements for the heat sources, cooling andelectromechanical arrangements can be combined to provide the efficientproduction of BNNTs. For example, see related applications U.S.Provisional Patent Application No. 62/164,997, International PatentApplication No. PCT/US14/063349, and international Patent ApplicationNo. PCT/US15/27570, which are incorporated by reference.

As described herein, synthesis of BNNTs, including high quality BNNTs,can be enhanced by management of the heat going into and out of thepreheat-support, BNNT melt, and self-assembly zones, the control of thenitrogen gas flow through the chamber in the process, and theinterfacing between the boron melt and the target holder. Heat may beapplied to one or more of the zones to enhance BNNT formation, and thetarget holder and support structures may be used to manage the flow ofheat and nitrogen gas through the chamber. Baffles and tube structuresmay be incorporated to control the flow of nitrogen gas to keep in thepreferred laminar or steady flow condition, e.g. not unsteady orturbulent flow conditions. Through implementing one or more of thepresent approaches, BNNT synthesis process may be enhanced, includingfor example the yield of high quality HTP BNNTs both in terms ofquality, e.g. highly crystalline with few defects and lengths greatly inexcess of several hundred microns, and in terms of quantity.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the approach. Asused herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The principles described herein may be embodied in other specific formswithout departing from the spirit or essential characteristics thereof.The present embodiments are therefore to be considered in all respectsas illustrative and not restrictive.

What is claimed is:
 1. An apparatus for synthesizing boron nitridenanotubes, the apparatus comprising: a chamber configured to receivenitrogen gas in a first direction; a target holder having a boronfeedstock contact surface and a post, the post supporting the targetholder in a desired position within the chamber; a heat source forheating a boron feedstock on the target holder; and at least one mirrorpositioned in the chamber and configured to reflect heat from the heatsource towards the target holder.
 2. The apparatus of claim 1, whereinthe target holder comprises at least one flow control baffle to modifythe nitrogen gas flow in the first direction.
 3. The apparatus of claim1, wherein the heat source comprises a laser having a beam path, andfurther comprising a plano-convex cylindrical lens positioned in thebeam path.
 4. The apparatus of claim 3, wherein the mirror comprises aplano concave reflector positioned in the beam path downstream of theplano-convex cylindrical lens and the target holder.
 5. The apparatus ofclaim 2, wherein the at least one flow control baffle is configured tomaintain laminar flow of the nitrogen gas near the contact surface. 6.The apparatus of claim 1, wherein the mirror is further configured toreflect black body radiation toward the target holder.
 7. The apparatusof claim 1, wherein the heat source is configured to heat a first sideof the boron feedstock, and the mirror is configured to reflect heattowards a second side of the boron feedstock.
 8. The apparatus of claim7, wherein the second side is substantially opposite the first side. 9.An apparatus for synthesizing boron nitride nanotubes, the apparatuscomprising: a chamber configured to receive nitrogen gas in a firstdirection; a target holder having a boron feedstock contact surface anda post, the post supporting the target holder in a desired positionwithin the chamber; a heat source for heating a boron feedstock on thetarget holder; at least one flow control baffle to modify the nitrogengas flow in the first direction; and at least one mirror positioned inthe chamber and configured to reflect heat towards the target holder.10. The apparatus of claim 9, wherein the heat source comprises a laserhaving a beam path, and further comprising a plano-convex cylindricallens positioned in the beam path.
 11. The apparatus of claim 10, whereinthe mirror comprises a plano concave reflector positioned in the beampath downstream of the plano-convex cylindrical lens and the targetholder.
 12. The apparatus of claim 9, wherein the at least one flowcontrol baffle is configured to maintain laminar flow of the nitrogengas near the contact surface.
 13. The apparatus of claim 9, furthercomprising at least one heat sink on at least one of the target holderand the post, the heat sink configured to increase the rate of heattransfer from the target holder.
 14. The apparatus of claim 9, whereinthe contact surface is a concave surface.
 15. The apparatus of claim 9,wherein the mirror is further configured to reflect black body radiationtoward the target holder.
 16. The apparatus of claim 9, wherein thetarget holder is comprised of at least one of copper, molybdenum,tungsten or an alloy of two of these materials.
 17. An apparatus forsynthesizing boron nitride nanotubes, the apparatus comprising: achamber configured to receive nitrogen gas in a first direction; atarget holder having a boron feedstock contact surface and a post, thepost supporting the target holder in a desired position within thechamber; a heat source for heating a target region on the target holder;at least one flow control baffle configured to maintain laminar flow ofnitrogen gas near the target holder; and at least one reflective surfacein the chamber configured to reflect heat toward the target region. 18.The apparatus of claim 17, wherein the reflected heat comprises at leastone of heat from the heat source and black body radiation from thetarget region.
 19. The apparatus of claim 17, wherein the heat source isconfigured to heat a first side of the target region, and the reflectivesurface is configured to reflect heat toward a second side of the targetregion.
 20. The apparatus of claim 19, further comprising a secondreflective surface in the chamber and configured to reflect heat towarda third side of the target region.